The German patent specification No. 2,050,220 describes a music instrument which is provided with mechanical or electroacoustic resonators in order to emphasize the formant intervals characteristic for the timbre of its sound.
On page 24 of issue No. 1 of "Funktechnik" 1974 there is a description of a fourpole for the conversion of white noise to pink noise while on pages 227 to 229 in issue No. 7 there is a description of a method of measurement for determining the trackability of sound pick-ups in which frequency analyzers are used. "Pink noise" is here defined as noise in for thirds which the energy level for tierces), or octaves, respectively, remains constant independently of the frequency, so that a frequency-independent source is obtained. On pages 323 to 325 of issue of No. 9 of the same year there is a description of calibrating phonograph records for the rapid termination of the characteristics and for checking the tracking properties of stereo pick-ups, together with a measurement circuit for determining the distortions.
U.S. Pat. No. 4,101,840 describes a circuit arrangement with a varying degree of response or transmission in the case of which the middle frequency of formant-like maxima of the amplitude/frequency curve of the transmission response is slightly changed in accordance with the volume or loudness setting and/or the amplitude ratio of the formant-like maxima is changed with the loudness setting.
The U.S. Pat. Nos. 2,235,249 and 3,719,782 describe methods and apparatus for acoustic control for electro-acoustic play-back systems.
In the phonograph and tape recorder industry there is a substantial interest to restore old recordings, that is to say both in the case of sung performances and also in the case of instrumental and orchestra recordings. A certain improvement in the acoustic quality of old sound recordings can be achieved by suppression of and compensation for noise in the upper frequency ranges; however the results are by and large not always satisfactory and in particular it has so far only been possible to overcome the effects of wear, which more particularly impair the forte passages of the middle and higher pitch ranges, to a limited extent.